(Courtesy to Microsoft Encarta for this photograph.)

Nebulae are made of gases and small amounts of dust. This is where the life of a star is started. The name "nebula" was first put on any stellar object that was hard to view, but when telescopes were invented, people started to call nebulae the pockets of gas and dust in space. Most of the time nebulae are invisible, but you know that they are there by the dark patches that sometimes cover up stars. Other times, nebulae are actually visible by other stars lighting up the gases.

There are different kinds of nebulae too. Planetary nebulae (or planetaries) are like planets when they are viewed through a telescope. A nebula that was formed when a star turned from a red giant to white dwarf forms a smaller planetary nebulae, and when a red supergiant dies in a supernova, it creates a much larger planetary nebula. One example of a supernova planetary nebula is the Crab Nebula. Usually the supernova planetaries are very radioactive and sometimes have pulsars in them. If you want to find out what pulsars are, please click here. The other kind of nebulae are called diffuse nebulae. They are the "original" nebulae in the universe. Diffuse nebulae usually have more gases and dust in them than the planetary nebulae do. These diffuse nebulae can be light-years long and have no definite shape. An example of a diffuse nebulae is the Great Nebula in the Orion constellation.

Stars form by gases and dust clumping together. This is because all matter contains gravity, and when the clumps of gas and dust get together, the pressure creates heat and the pockets of gas and dust turn into a protostar. This process takes a few millions of years, only a very small portion of a star's life. Sometimes if protostars spin very quickly, it may split into two protostars. This forms a binary star system. Scientists can tell whether or not there is a black hole in a binary star system because of how the black hole affects the other star. As the protostar continues in life, it continually spews out jets of hot gas. This drives away any remaining gas and dust. This gas and dust goes in orbit around the protostar and may form planets later on. When the core of a protostar reaches 18 million °F (10 million ° C), a star is born. Afterwards, the star continues to have nuclear fusion and it may live for many billions of years.

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